1. Field of the Invention
This invention relates to a rotation drive system having a speed reduction device with elastic bodies, and for example, relates to a rotation drive system having a speed reduction device with elastic bodies that reduce the rotational speed of a motor, which rotates a photoreceptor drum, transfer belt and other components in a copying machine and so forth, by means of frictional contact of the elastic bodies.
2. Description of Background Art
FIG. 3 is a cross-sectional view illustrating a conventional rotation drive system. This rotation drive system is disclosed in Japanese unexamined patent publication No. 2002-115751. In FIG. 3, the rotation drive system includes a motor 1, a speed reduction device 2 and a speed detection mechanism 3. The speed reduction device 2 employs a traction system (friction transmission system), which is considered advantageous in reducing the rotational variations.
An end of a rotary shaft 14 in the motor 1 acts as a sun roller 20 and makes contact with a plurality of planetary rollers 22. Each planetary roller 22 is cantilevered by a rod 28 from a carrier 23. The planetary rollers 22 are in contact with an inner surface of an internal ring 21 via elastic bodies 31 such as rubber. The rotation of the rotary shaft 14 driven by the motor 1 produces torque which is reduced by the sun roller 20, planetary rollers 22 and internal ring 21 at their reduction ratios depending on the individual external and internal diameters, and the torque is then output through the carrier 23 and an output shaft 24.
The output from the speed detection mechanism 3 is input to a controller 4. Based on a control signal from the controller 4, a drive device 5 controls the rotational speed of the motor 1. Since the planetary rollers 22 used in the speed reduction device 2 make contact with the internal ring 21 via the elastic bodies 31 which may cause generation of a delay element in a feedback control loop, the controller 4 should so control the drive device 5 as to prevent it from being uncontrollable as a result of the delay element.
For this purpose, the controller 4 adopts a feedback control as disclosed in Japanese unexamined patent publication No. 2002-171779. Specifically, a rotational speed output from the speed reduction device 2 is detected to obtain a difference value from a target speed. The controller 4 feeds a speed command signal based on the difference value to the motor 1 to directly control the motor's rotational speed, thereby reducing delay factors.
In the rotation drive system shown in FIG. 3, a motor 1 acting as a driving source is a stepper motor. The speed of the stepper motor can be controlled by controlling the number of pulses applied to the stepper motor. In addition, torque can be controlled by adjusting current passing through motor coils.
However, the application of load torque that is equal to or greater than torque produced by the motor to the stepper motor could cause a phenomenon, so-called “out-of-step”, which stops the movement of the motor at the occurrence of the phenomenon and from then on. This is a drawback of the stepper motor. The term “out-of-step” herein means being out of synchronization.
In order to properly drive the stepper motor, in consideration of variations in the amount of load, the current must be set so that the motor produces torque equal to or greater than an estimated actual amount of load torque. Thus, the wide variation of the load torque requires setting a considerable amount of wasted current. The increase of the current value may adversely affect motor properties including electricity consumption, heat generation, noise, vibration, rotational variation, and so forth.